Acryloxysilanes and methacryloxysilanes are chemically reactive materials which are useful in many commercial applications. For example, such compounds are useful as coupling agents to bond organic compounds to inorganic materials. In particular, 3-methacryloxypropyltrimethoxysilane is widely used as a coupling agent in enhancing the performance of fiberglass-reinforced products.
Acryloxysilanes and methacryloxysilanes can be prepared by the known reaction between organosilicon compounds having an Si--H functional group and acryloxy and methacryloxy compounds having additional aliphatic unsaturation. For example, 3-methacryloxypropyltrimethoxysilane (sometimes herein referred to for brevity as "MAOP-TMS") can be prepared by the known reaction of allyl methacrylate with trimethoxysilane, as shown by the following equation (1): ##STR1## Likewise, reaction of allyl methacrylate with trichlorosilane, H--SiCl.sub.3, provides 3-methacryloxypropyltrichlorosilane which in turn can be reacted with methanol to produce MAOP-TMS. When allyl acrylate is used in place of allyl methacrylate, the corresponding acryloxypropyltrimethoxy- (or trichloro-) silanes are provided. Due to the exothermic nature of such hydrosilation reactions, polymerization of the highly reactive acryloxysilane and methacryloxysilane product can occur as product is formed. Such polymerization can also be induced during esterification of the trichlorosilane intermediate to the corresponding trialkoxysilane product, such as, for example, during the aforementioned reaction of 3-methacryloxypropyltrichlorosilane with methanol to produce MAOP-TMS.
Undesired polymerization can also occur during purification of the crude reaction product. Typically, purification is accomplished by distillation, which is preferably carried out at as low a temperature as feasible to minimize polymerization. Even purified product may tend to polymerize during storage prior to end use.
Depending on the extent of such polymerization during initial formation, purification and storage of acryloxy- and methacryloxysilanes, thickening and even gelling may occur, resulting in increased maintenance to remove the thickened or gelled material from equipment or in unsalable product.
Various approaches are known to the art for minimizing undesired polymerization of acryloxy- and methacryloxysilanes during their manufacture. One such approach, described in U.S. Pat. No. 4,276,426 to Lindner et al., comprises continuously charging the reactants to a pipe-shaped reactor, and continuously circulating the reaction mixture at a velocity of at least 1000 centimeters per minute. In one embodiment of this patent, allyl methacrylate, trichlorosilane and a platinum catalyst are continuously circulated at a velocity of 3500 centimeters per minute. The patentees report that when the same reaction is repeated without circulation, "the contents of the reactor jelled" after one hour (column 5, lines 13-16). It is evident that this technique for avoiding polymerization during the hydrosilation reaction requires high speed continuous operation and is susceptible to gelling as a result of process fluctuation.
Another approach to producing stable acryloxy- and methacryloxysilanes is to employ polymerization inhibitors such as those disclosed in: U.S. Pat. No. 3,258,477 to Plueddemann et al.; U.S. Pat. No. 4,709,067 to Chu et al.; and U.S. Pat. No. 3,816,267 to Chuang.
The Plueddemann et al. patent describes a variety of reactions for preparing acryloxy- and methacryloxysilanes including hydrosilation, illustrated by above equation (1). In effecting such reactions, Plueddemann et al. state the desirability of using "polymerization inhibitors such as copper acetate and hydroquinone to prevent polymerization of the silane product by way of the acrylate double bond" (column 3, lines 66-69). In illustrating the formation of 3-acryloxy- and 3-methacryloxypropylsilanes by the platinum-catalyzed hydrosilation of allyl acrylate and allyl methacrylate with trimethoxy- or tricholorosilane, Plueddemann et al. employ 2,5-ditertiary butyl hydroquinone (Examples 1 and 2) and hydroquinone (Example 9) as polymerization inhibitors at levels exceeding 1000 parts by weight per million parts by weight (ppm) of silane product. Another reaction described by Plueddemann et al. for producing acryloxy- and methacryloxysilanes comprises the platinum-catalyzed reaction of a tertiary-amine salt of acrylic or methacrylic acid with a chloroalkylsilane. In effecting this type of reaction, Plueddemann et al. state: "It is also best to carry out the reaction in the presence of one or more polymerization inhibitors for acrylic or methacrylic acid, such as hydroquinone and N,N'-diphenylphenylene diamine" (column 4, lines 16-19). In illustrating this particular reaction using triethylamine, methacrylic acid and chloromethyltrimethoxysilane Plueddemann et al. employ in their Example 5, hydroquinone as the inhibitor, again at a high level of at least 1000 ppm of the methacryloxytrimethoxysilane product.
Further, U.S. Pat. No. 4,709,067 to Chu et al. discloses a multi-step process for preparing acryloxy- and methacryloxysilanes. The first three steps involve: charging an inhibited acryloxy or methacryloxy functional compound to a first reservoir; charging an Si--H compound and platinum catalyst to a second reservoir; and combining the contents of the two reservoirs in a reactor in which the hydrosilation is effected. In addition to the presence of inhibitor during the hydrosilation reaction, additional inhibitor is added to the crude reaction product prior to vacuum distillation.
More specifically, Chu et al. employ phenolic inhibitors such as the monomethyl ether of hydroquinone, Ionol.TM. and Isonox.TM. 129; aromatic amines such as diphenylene diamine; and aromatic sulfur compounds such as phenothiazine. (Ionol.RTM. is 2,6-di-tert-butyl-4-methyl phenol; ACS SOCMA Handbook, p. 63A, 1965. Isonox.TM. 129 is 2,2'-ethylidenebis[4,6-di-tert-butylphenol]; ACS On-Line File Registry of Chemical Trademarks, RN 35958-30-6, 1991). In the preferred embodiment of Chu et al., only the phenolic inhibitors are used in the hydrosilation reaction step and a combination of both phenolic and non-phenolic inhibitors is used during vacuum distillation. The concentration of inhibitor used during hydrosilation varies from 0.2 to 5.0% by weight (2,000 to 50,000 ppm) of the silylated acrylate or methacrylate product. During vacuum distillation the concentration of the non phenolic inhibitor varies from 200 to 10,000 ppm, while the concentration of phenolic inhibitor varies from 500 to 15,000 ppm, based on the weight of methacryloxysilane product distilled.
The aforementioned U.S. Pat. No. 3,816,267 to Chuang relates to the distillation of acrylates and methacrylates in the presence of certain polymerization inhibitors. Chuang employs as inhibitors a mixture of a quinone and an enol derivative of a quinone in an amount sufficient to prevent polymerization. Preferably, at least 100 ppm of each inhibitor is used during the distillation process. Lower inhibitor levels (i.e. 10 ppm) can be utilized " . . . if the temperature within the column is low, and one employs reduced pressure, and the distillation is not prolonged . . . " (Column 3, lines 52-57).
In addition to stabilization of acrylates and methacrylates, Chuang discloses the applicability of his polymerization inhibitors to acryloxy- and methacryloxysilanes. For example, at column 4, lines 46-44, Chuang states:
" . . . the methacrylate and acrylate esters which have a hydrolyzable silicon group bonded to the ester moiety, such as a trimethoxysilyl moiety, represent the most difficult to stabilize from distillation when distilled from a crude mixture containing them. The basis for this is the belief that during distillation, heat causes a certain number of the hydrolyzable groups, such as, for example, methoxy groups, to be cleaved from the silicon atom and act as a free radical which induces polymerization at a lower temperature than would be normally expected for such an acrylate or methacrylate molecule. Hence, a severe test for inhibiting the polymerization of acrylate and methacrylate esters would be the stabilization of such methacrylato or acrylato silane esters . . . . "
In illustrating the applicability of his dual inhibitor system to 3-methacryloxypropyltrimethoxysilane, Chuang employs total inhibitor levels ranging from 100-700 ppm during the distillation thereof.
As is evident from the above discussion of the Plueddemann et al., Chu et al. and Chuang patents, the state of the art is such that high levels (at least 1000 ppm) of polymerization inhibitors are employed in the initial formation of acryloxy- and methacryloxysilanes and that, even during their purification by vacuum distillation, at least 100 ppm of inhibitor is used. Such levels of inhibitor can have adverse effects on product properties such as color. A color problem, for example, can be reduced by lowering the level of inhibitor used. Unfortunately, with reduced inhibitor levels the risk of undesired product polymerization increases.
A still further approach for dealing with the polymerization problem is described in U.S. Pat. No. 4,780,555 to Bank. This patent discloses a method for stabilizing acryl-functional halosilanes made by reacting a halosilane having an Si--H group with an acryloxy- or methacryloxy-functional organic compound in the presence of a platinum catalyst. The acryl-functional halosilane is inhibited with 100 to 2,000 ppm of phenothiazine while sparging the reaction mixture with a gas composition containing at least 0.1%, preferably from 2 to 4%, by volume oxygen in an inert gas such as nitrogen. A drawback of this method is that the disclosed inhibitor system is effective only when the specified oxygen level is present.
Accordingly, a need exists to stabilize acryloxy- and methacryloxysilanes against polymerization with an inhibitor which is capable of inhibiting polymerization at low concentrations and which is effective under aerobic as well as non-aerobic conditions and does not require special process features to be effective. It also is desirable that such an inhibitor not cause discoloration of the inhibited product, even when the inhibitor is used in relatively high concentrations.